Cell surface receptors mediate the biological actions of growth factors. Most growth factor receptors are activated by ligand induced homo or heterodimerization. Because of their role in regulating cellular functions and the diseases caused by their disregulation, much interest exists in the pharmaceutical industry to develop drugs targeting growth factor receptors. In this grant, we propose to develop a novel technology that can be used to discover small molecule growth factor antagonists or agonists. The assay is based on our enzyme fragment complementation (EFC) technology that utilizes two genetically engineered fragments of E. coli [unreadable]-gal. The larger fragment, Enzyme Acceptor (EA), contains a deletion near the amino terminus, while the smaller fragment, ProLabel, contains the amino-terminal sequence missing from EA. Alone, EA is inactive, but in vitro it can spontaneously recombine with ProLabel to form an active enzyme that can catalyze the formation of a luminescent product that can be detected photometrically. We have begun to adapt EFC to measure the interaction of different growth factor receptor subunits. The spontaneous, robust complementation of EA and ProLabel was transformed into a protein interaction detection system by mutating the ProLabel peptide to create a weakly complementing [unreadable]-gal fragment termed ProLink. Because the system complements only weakly, when EA and ProLink are genetically fused to two proteins that interact, the interaction of the two proteins enhances the complementation resulting in increased [unreadable]-gal activity. In this grant, we will provide proof of principle of the universality of the assay by developing EFC to measure activation of different subfamilies of growth factor receptors. These studies will provide the foundation of future phase II studies to optimize the assay format for HTS and initial drug discovery efforts against these different receptor subfamilies. This grant is focused on developing an assay that can measure the activation of any growth factor receptor in the Human Genome. This technology will allow for the discovery of novel small molecule drugs that regulate this important class of proteins and could provide for the generation of new drugs to treat a host of diseases including cancer and autoimmune disorders. [unreadable] [unreadable] [unreadable]